Tool with a molybdenum sulfide coating and method for producing the same

ABSTRACT

The invention relates to a tool, especially a cutting insert for cutting metallic materials, consisting of a hard metal, cermet, ceramic or steel base body and at least one coating deposited thereon. This single coating or in the event that there are several layers, the outer layer or the layer below the outer layer, consists essentially of molybdenum sulphide. The invention also relates to a method for producing the molybdenum sulphide coating by chemical vapour deposition. According to the invention, the molybdenum sulphide coating contains a mixture of the sulphide phases MoS 2  and Mo 2 S 3  with an essentially random orientation (isotropic orientation) of the phase crystals. The coating is applied by means of a chemical vapour deposition procedure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/DE99/03781 filed Nov. 30,1999 and based upon German national application 199 09 372.5 of Mar. 3,1999 under the International Convention.

FIELD OF THE INVENTION

The invention relates to a tool, especially a cutting insert, for themachining of metallic workpieces, composed of a hard metal base body, acermet base body, a ceramic base body or a steel base body and at leastone layer deposited thereon, in which the sole layer or in the case of aplurality of layers, the outermost layer or the layer lying beneath theoutermost layer is constituted predominantly of molybdenum sulfide.

The invention also relates to a method of producing the molybdenumsulfide layer by means of CVD (chemical vapor deposition).

BACKGROUND OF THE INVENTION

The coating of substrate bodies of hard metal, a cermet, a ceramic, orsteel with hard materials or hard material mixtures or ceramic materialsis known from the state of the art. Especially, carbides, nitrides,carbonitrides, oxycarbonitrides and/or borides of at least one of theelements of groups IVa to VIa of the Periodic Table are used, such as,for example, TiC, TiN, Ti(C,N) or also ceramic materials like Al₂O₃ orZrO₂. With large cutting speeds and/or large chip cross sections, hightemperatures arise at the cutting edge of the machining tool especiallyin the case of materials which are difficult to machine, thereby givingrise to increased wear which can lead in part also to edge breakage. Tominimize the temperatures which arise at the cutting edge of the toolduring machining, usually so-called cooling lubricants have been usedwhich contain either toxic and environmentally polluting substances andwhich must be disposed of at relatively high cost.

A further possibility, which can reduce the use of coolant lubricantsdrastically, is the principle of so-called minimum quantity lubrication,whereby very small amounts of coolant lubricants are supplied with pointprecision through fine passages to the location between the tool cutterand the workpiece. Apart from a high cost for carrying out the methodand for the apparatus involved, this approach does not eliminate thewaste treatment problem but only minimizes it.

On the other hand, there are so-called solid dry lubricant films, forexample, molybdenum disulfide, known in the art. For example DE-A- 24 15255 proposes to improve the adhesion of a layer which is preferablybased upon MoS₂ or WS₂ and which is applied by so-called sputtering witha thickness of 1 to 2 μm and in which the application of a dry lubricantlayer based also upon MoS₂ or WS₂ can be effected in a conventionalmanner by a polishing application. In the East German patent document(DD) 202 898 it has been proposed to apply layers or the like ofmolybdenum disulfide by sputtering to cutting tools, stamping tools,drawing tools and machining tools, whereby the layer thickness lies inthe nm range. The layer which is applied by sputtering to a substratebody provided with a hard material layer should have a hexagonal latticestructure. EP 0 534 905 A2 also proposes the PVD (plasma vapordeposition) coating of machining tools, inter alia with molybdenumdisulfide.

It has, however, been found that MoS₂ layers produced by cathodicatomization (sputtering) adhere poorly to the substrate body or to theouter hard material layer applied to the substrate body or have apredominantly unsatisfactory orientation of the MoS₂ crystallites. Toobtain the lowest friction coefficients, the hexagonal components of theMoS₂ layer should be so oriented that the hexagonal base planes lieparallel to the surface. Thereby the oxidation resistance of the layeris also improved.

It is thus proposed in DE 35 16 933 A1 that the ratio of the water vaporpartial pressure to the coating rate be adjusted to be smaller than 10⁻⁷mbar/1.6 mgm⁻²s⁻¹ which, in practice, can only be achieved withconsiderable technological cost.

Apart from the requirement which is customary for the PVD process forpurity, i.e. that the gas atmosphere not contain foreign substances,there is a further drawback with the PVD process in that a directedparticle stream is provided from the target source to the substratewhereby the substrate must be rotated about three axes to produce auniform coating.

Furthermore WO 97/04142 describes a process for uniform coating of MoS₂and a metal or a metal compound like titanium or titanium nitridethrough the use of two targets by means of a sputtering process.

While MoS2 is relatively soft and is not especially wear-resistant, theformation of an MoS₂ coating has a significant effect since by the useof a single MoS₂ layer or an outermost MoS² layer at the contactlocation of the tool with the workpiece, the MoS₂ may be quickly abradedbut the friction force because of the penetration of the MoS₂ particlesfrom the edge inwardly (entrainment) is still noticeably reduced for aperiod of time.

OBJECTS OF THE INVENTION

It is the object of the present invention to provide a single layer oran outermost layer or layer combination which has a dense latticestructure and a higher hardness of greater mechanical stability againstwear. It is also an object to provide a method of producing such alayer.

DESCRIPTION OF THE INVENTION

These objects are achieved by a tool in which the molybdenum sulfidelayer contains a mixture of the sulfide phases MoS₂ and MoS₂S₃ with asubstantially random orientation of the phase crystals. The layeredlattice structure is comprised of randomly oriented MoS₂ grains whichpartly have a platelet-shaped configuration. The intervening spaces arefilled with the Mo₂S₃ phase. With these features a surprisingly improvedwear resistance of the layer is achieved on machining.

Thus still further phases of molybdenum sulfur compounds and/or chlorinecan be contained in the molybdenum sulfide layer but should not exceed 6atomic %.

A single molybdenum sulfide layer or the outermost molybdenum sulfidelayer can also be fabricated so as to be of a multilayer type, wherebythe respective undermost layer or an underlying layer with a thicknessrespectively not exceeding 200 nm has a hexagonal lattice structure witha preferred orientation of the hexagonal planes parallel to the toolsurface. The remaining layers however are of isotropic orientation. Thisapplies especially for a substrate body which is coated with only asingle molybdenum sulfide layer (and no other layer). The layer with thehexagonal MoS₂ crystals is thereby bounded directly by the substrate andhas a thickness of 100 nm to 200 nm.

According to a further feature of the invention, the Mo₂S₃ component inthe molybdenum layer does not remain continuously homogeneous but withincreasing distance from the substrate body surface is present indifferent proportions, preferably varying periodically from 5 to 30volume %.

According to a further feature of the invention, the molybdenum sulfidelayer contains successive individual layers of different compositions ofwhich one type comprises the mixture of MoS₂ and Mo₂S₃ sulfide phasesand the other type consists substantially of pure MoS₂. Each of the MoS₂layers has a maximum thickness of 200 nm. The thicknesses of layers ofthe mentioned phase mixture (MoS₂/Mo₂S₃) respectively lie between 200 nmand 400 nm.

In a further embodiment of the tool of the invention there are stillfurther metallic phases and/or hard material phases in the molybdenumsulfide layer under consideration and whose volume proportion however ispreferably not in excess of 10 volume %. The metals in question can beespecially molybdenum and the materials especially molybdenum carbide,titanium carbonitride or chromium carbide. Such metal phases or hardmaterial phases serve to increase the stability of this layer whichhowever is in substance the molybdenum sulfide of the mixture describedat the outset. The volume proportion of MoS₂ in this layer lies between30 and 98 volume %, preferably between 40 to 95 volume %. The layerthickness lies between 0.5 to 8 μm, preferably between 2 and 4 μm.

Optionally the single or outermost layer of molybdenum sulfide can havestill a further metallic layer of a thickness ≦200 nm applied thereto.Preferably metals of groups IVa to VIa of the Periodic Table are used.Alternatively these thin outer layers of a maximum of 200 nm inthickness can also be composed of a hard material, preferably a carbide,a nitride or a carbonitride of one of the aforementioned elements.

The molybdenum sulfide layer is applied by means of a CVD process,preferably using the subsequently described process technique.

For producing the mentioned coating, a CVD process is used in which thegas mixture is composed of molybdenum chloride, hydrogen sulfide, anoble gas and hydrogen and with a coating temperature lying between 500°C. and 900° C., preferably between 600° C. and 850° C. The pressureamounts to 0.1 to 50 kPa, preferably between 0.1 and 1 kPa. The molarratio between hydrogen sulfide and molybdenum chloride lies between 0.1and 4, preferably between 0.1 and 2. The proportion of hydrogen andargon together makes up between 80 to 99 volume %, preferably between 90to 98 volume %.

Preferably MoCl₅ is used although optionally MoCl₄ or mixtures of MoCl₅and MoCl₄ can be used.

The maintenance of the above-mentioned parameters is significant for theproduction of the layer consisting essentially of molybdenum sulfide.With greater than the aforedescribed molar ratios, layers are obtainedwhich consist exclusively or predominantly of hexagonal MoS₂. MoCl₅ candecompose at temperatures above 200° into MoCl₄ and Cl so that themolybdenum sulfide can be formed both from MoCl₅ and also from MoCl₄.

The mixed layer, which contains hexagonal MoS₂ and Mo₂S₃ can on the onehand be produced as a coating with a constant Mo₂S₃ proportion andrandom orientation of the MoS₂ crystallites. On the other hand themethod of the invention can also produce multilayer coatings which canhave alternating layers of MoS₂/Mo₂S₃ phase mixtures and layers oforiented MoS₂. In the last-mentioned case, the MoS₂ layers howevershould not be thicker than 200 nm since otherwise pores will develop inthe molybdenum sulfide layer whereby the density thereof will diminish.In a special embodiment, the molar ratio H₂S/MoCl₅ during the coatingalternates for equally long time periods in which one of the selectedmole ratios is greater than the other by a factor of 7 to 10 so thatwith a greater mole ratio, pure MoS₂ is deposited and with the smallermole ratio, a phase mixture of MoS2 and Mo₂S₃ is deposited. The durationover which the substantially pure MoS₂ is deposited is that which willprovide a maximum permissible layer thickness of 200 nm. By comparison,the duration in which the mole ratio is set to deposit the phase mixtureMoS₂/Mo₂S₃ can be two or three times as great. Preferably 8 to 12 layersare deposited of which substantially half are composed of the mentionedphase mixture and the other half are of MoS₂.

Alternatively here too, the molar.ratio H₂S/MoCl₅ can be so controlledthat during deposition the Mo₂S₃ phase proportion periodically varies.

The molybdenum sulfide layer is composed in essence of hexagonal MoS₂and Mo₂S₃ although it can however contain still further phases which canbe formed from the gas mixture of MoCl₅, H₂S, H₂ and argon. In thismanner, small proportions of rhombo-hedral MoS₂, MoS₂Cl₂ and MoCl₃ canarise which, however, have no noticeable effect on the layercharacteristics.

The layer consisting essentially of molybdenum sulfide can also beformed in a multilayer manner whereby the individual layers containingessentially MoS₂ and Mo₂S₃ can be separated by intermediate layers of ametal or a metallic hard material. The intermediate layers are howeveralways thinner than 0.3 μm. As materials for such intermediate layers,molybdenum or molybdenum carbide are to be considered, these enhancingmechanical stability of the molybdenum sulfide layer according to theinvention. To deposit molybdenum or molybdenum carbide, the gas mixturecomposition in the CVD process is so altered that the feed of H₂S isinterrupted and replaced by CH₄. By rapid switching of the gas it ispossible to obtain quasicontinuous layers which aside from MoS₂ andMo₂S₃ also contain other phases. With this feature, the stability of thedry lubricant layer can also be improved. In practice one balances whichcomponents of the stability enhancing nonsulphidic impurities arepresent or intermediate layers are used. The lowest frictioncoefficients are obtained with layers which are comprised of hexagonalMoS₂ but can have the stability by the additional Mo₂S₃ phase andfurther by nonsulphidic impurities. The choice of the respectiveproportions of stabilizing-enhancing compounds and/or metals of hardmaterials depends upon the kind of workpiece material to be machined andthe cutting forces arising during machining. The invention is describedin further detail using concrete examples:

SPECIFIC EXAMPLES Exemplary Embodiment 1

A CVD apparatus with a heatable quartz glass tube is used as thereaction chamber. The MoCl₅ gas is generated by evaporation of MoCl₅salt at 110° C. in an argon flow and fed to the reaction chamber heatedto 650° C. In a second test, the other gases are fed. The gas mixture iscomprised of 0.4 volume % H₂S, 2.2 volume % MoCl₅, 14.6 volume % H₂ and82.8 volume % Ar. The molar ratio H₂S/MoCl₅ amounts to 0.18. During thecoating a pressure of 500 Pa is maintained in the reaction chamber. Inthe chamber, there are to be found uncoated lathe cutting insert platesand cutting insert plates coated with titanium nitride/titaniumcarbonitride/titanium nitride and thin silicon disks for analytic tests.After the coating duration of 150 minutes, layers of about 3 to 4 μmthickness are formed with a very dense lattice structure. By phaseanalysis by means of X-ray diffraction and electron diffraction,hexagonal MoS₂ and MO₂S₃ are found. The ratio in terms of quantity ofthe two phases can be regarded based upon the visibility of the twophases and is relatively coarsely defined. The volume proportion of thehexagonal MoS₂ phase amounted in the present case to about 60%. Therelatively thick layer is adhesive and scratch resistant. The Vickershardness amounted to 400 HVO₂. In tribological tests a coefficient offriction of 0.11 was found (against a steel ball).

Exemplary Embodiment 2

In another test with the above-described apparatus, two gas mixtures areused alternately in which the molar ratio H₂S/MoCl₅ is varied by afactor of 10. At the beginning of the test, a gas mixture of 0.2 volume% H₂S, 0.9 volume % MoCl₅, 14.8 volume % H₂ and 84.1 volume % Ar issupplied. The deposition temperature is 670° C. and the pressure 500 Pa.Under these conditions, a phase m mixture of hexagonal MoS₂ and MO₂S₃ isdeposited. After 5 minutes, the H₂S gas flow is increased by aboutten-fold while all other gases are held constant. The new gas mixture of1.8 volume % H₂S, 0.9 volume % MoCl₅, 14.6 volume % H₂ and 82.7 volume %Ar is fed to the reactor for a duration of two minutes. Under theseconditions essentially pure MoS₂ is deposited. The alternation betweenboth gas mixtures is effected 10 times. With the switchover times, thetotal coating interval was about 80 minutes. A dense layer with athickness of 3.2 μm was obtained. In an X-ray diffractogram it can beseen that the intensity of the (0.02) reflection of the hexagonal MoS₂is significantly greater than in the first embodiment. This means thatmore of the hexagonal MoS₂ is significantly greater than in the firstembodiment. This means that more of the hexagonal MoS₂ oriented parallelto the surface normal is present. The bases for observation are thetextures of the single phase MoS₂ layers. The Vickers hardness of thelayer amounted to 380 HVO2 and the friction coefficient to 0.08.

Exemplary Embodiment 3

A temperature of 800° C. and a pressure of 500 Pa is established and thegas mixture of 0.4 volume % H₂S, 2.2 volume % MoCl₅, 54.2 volume %H_(2 g)and 43.2 volume % Ar is supplied. After five minutes the H₂S isshut off and all remaining parameters are not changed. From the MoCl₅,H₂ and Ar a thin layer of molybdenum metal is formed over five minutes.After 110 minutes, an about 4 μm thick layer with the phases MoS₂ andMo₂S₃ is formed which has an enhanced molybdenum content and a denslattice structure. The hardness value of the layer amounted to 520 HVO₂and the friction coefficient to 0.14.

Exemplary Embodiment 4

Tests were carried out in Example 3 but with methane in the second stepinstead of H₂S: 6.9 volume % CH₄, 1.8 volume % MoCl₅, 55.3 volume % H₂and 36 volume % Ar. This variant gave rise to the formation of thinintermediate layers of Mo₂C. The hardness value of the layer amounted to530 KVO2 and the friction coefficient to 0.18.

The dry lubricant layers were tested using commercial lathe cuttingplates of the form CNMG120412 which were constituted of a hard metalalloy of 6% Co, 8% (Ti,Ta,NB)C and 86% WC. The plates were provided withabout 12 μm thick coating of the three layers TiN, Ti(C,N) and TiN whichwere applied in a CVD production apparatus at about 1000° C. Such plateswere additionally provided with coatings according to the invention inthe above-described test apparatus. In these examples layers withapproximately the same thickness (3 μm) of the outermost MoS₂/Mo₂S₃ wereused as outermost layers. With these coating plates, a shaft ofstainless steel was turned in smooth sections until the cutting cornerbecame unusable because of wear or breakage. The turning tests utilizeddry cutting with cutting speeds of 200 m/min, cutting depths of 2.5 mmand a feed of 0.25 mm per revolution. The following results wereobtained:

Cutting body CNMG120412,

Coated with TiN/Ti (C,N)TiN

Thickness of the MOS₂/Mo₂S₃- Embodiment Life Containing Layer (Minutes)Without an outer layer 0 μm  9 according to the invention Dry lubricantlayer 3.0 μm 17 according to Exemplary Embodiment 1 Dry lubricant layer3.2 μm 19 according to Exemplary Embodiment 2 Dry lubricant layer 3.2 μm16 according to Exemplary Embodiment 3 Dry lubricant layer 2.9 μm 15according to Exemplary Embodiment 3

These results show that all variants of the coating of the inventionincrease the use time in dry machining.

What is claimed is:
 1. A method of producing a molybdenum sulfidecoating on a cutting insert for machining metal, comprising the stepsof: (a) depositing on a cutting insert body of a material selected fromthe group which consists of hard metal, cermet, ceramic and steel as asole layer or as the outermost layer or a layer underlying the outermostlayer of a multiple layer coating a molybdenum sulfide layer containinga mixture of the sulfide phases MoS₂ and Mo₂S₃ by chemical vapordeposition from a gas mixture of molybdenum chloride, hydrogen sulfide,a noble gas and hydrogen; (b) maintaining a temperature during thechemical vapor deposition of 500° C. to 900° C. during the chemicalvapor deposition; (c) maintaining a pressure between 0.1 kPa and 50 kPaduring the chemical vapor deposition; (d) maintaining a molar ratio ofhydrogen sulfide/molybdenum chloride between 0.1 and 4 during thechemical vapor deposition; (e) maintaining a total of the hydrogen andnoble gas content between 80 and 99 volume percent during the chemicalvapor deposition; and (f) varying the molar ratio of hydrogensulfide/molybdenum chloride alternately for respective time intervalsfrom a low molar ratio to a high molar ratio at 7 to 10 times the lowmolar ratio so that with the high molar ratio pure MoS₂ is deposited andat the low molar ratio a phase mixture of MoS₂ and Mo₂S₃ is deposited.2. A The method according to claim 1 herein the duration over which themolar ratio is set so that essentially the phase mixture MoS₂/Mo₂S₃ isdeposited is two to three times as great as the duration in which theessentially pure MoS₂ is deposited.
 3. The method according to claim 1wherein 8 to 12 layers are deposited.
 4. The method according to claim 1wherein said temperature is controlled to be 600 to 850° C., saidpressure is set to be 0.1 to 1 kPa, said molar ratio lies between 0.1and 2, and the total of the hydrogen and noble gas content is set to bebetween 90 and 98 volume percent.
 5. A tool for machining of metalworkpieces, comprised of a base body of a material selected from thegroup which consists of hard metals, cermets, ceramics and steel andwherein a sole layer or in a case of multiple layers the outermost layeror a layer lying under the outermost layer is composed essentially ofmolybdenum sulfide, the molybdenum sulfide layer containing a mixture ofthe sulfide phases MoS₂ and MO₂S₃ with an essentially random andisotropic orientation of the phase crystals.
 6. The tool according toclaim 5 wherein the molybdenum sulfide layer contains othermolybdenum-containing, sulphur-containing or chlorine-containing phases.7. The tool according to claim 5 wherein the sole molybdenum sulfidelayer or the outermost molybdenum sulfide layer is multilayered and arespective lowermost layer or lowermost layers with respectivethicknesses not exceeding 200 nm have a hexagonal lattice structure witha preferred orientation of the hexagonal planes parallel to the toolsurface.
 8. The tool according to claim 5 wherein the Mo₂S₃ content fromthe interior outwardly with increasing distance from the substrate bodysurface, varies periodically by 5 to 30 volume %.
 9. The tool accordingto claim 7 wherein the successive layers of the molybdenum sulfide layerconsist of essentially pure MoS₂ and of a phase mixture of essentiallyhexagonal MoS₂ and Mo₂S₃, whereby each of the individual MoS₂ layers hasa maximum thickness of 200 nm and 400 nm.
 10. The tool according toclaim 9 wherein the single layer or the outermost or next to theoutermost layer which essentially comprises molybdenum sulfide, containstill further metallic phases with a volume proportion of ≦20% in theform of molybdenum or hard material phases.
 11. The tool according toclaim 10 wherein said volume proportion is ≦10% and said furthermetallic phases are selected from the group which consists of molybdenumcarbide, titanium carbide and chromium carbide.
 12. The tool accordingto claim 5 wherein the volume proportion of MoS₂ in the molybdenumsulfide-containing layer is between 30 and 98 volume %.
 13. The toolaccording to claim 12 wherein the volume proportion of MoS₂ is 40 to 95volume percent.
 14. The tool according to claim 5 wherein the thicknessof the essentially molybdenum sulfide-containing layer is 2 to 4 μm. 15.The tool according to claim 5 wherein the outermost layer has athickness ≦200 nm and is composed of a metal selected from an element ofGroups IVa to VIa of the Periodic Table and a carbide, nitride andcarbonitride of an element of Group IVa to VIa of the Periodic Table.